2015. 56, 6 UDC 541.6:546.3 MICROMECHANISM OF Cu AND Fe ALLOYING PROCESS ON THE MARTENSITIC PHASE TRANSFORMATION OF NiTi-BASED ALLOYS: FIRST-PRINCIPLES CALCULATION J.Y. Yin1, G.F. Li1,2, Y.L. Si1, G. Ying1, P. Peng2 1National Defence Key Discipline Laboratory of Light Alloy Processing Science and Technology, Nanchang Hangkong University, Jiangxi, P. R. China E-mail: lgf_918@126.com 2School of Materials Science and Engineering, Hunan University, Hunan, P. R. China Received July, 3, 2014 Using first-principles pseudo-potential plane wave method, the formation enthalpy 9H, binding energy 9E, elastic constants, and electronic structure were calculated and analyzed carefully for NiTiX (X = Cu, Fe) shape memory alloy. <...> The results show that the Cu or Fe element prefers to occupy the Ni site in the NiTi matrix phase respectively. <...> Compared with the NiTi matrix phase, the 9H, 9E, c44 and c of NiTi (Cu) are similar to each other. <...> However, the structural stability of the NiTi phase is improved obviously by the Fe alloying process. <...> Simultaneously, the shear modulus c44 and c of NiTi (Fe) are larger than those of the NiTi matrix phase. <...> Furthermore, Milliken population results indicate that QCu—Ti is smaller than QNi—Ti after the Cu alloying process, but QFe—Ti is larger than QNi—Ti. <...> The electron density difference shows that some covalent bonding exists between Fe and Ti elements. <...> Based on the upward analysis, the difference in the phase stability and elastic constants of NiTiX (X = Cu, Fe) is the substantial mechanism for the different Ms of NiTiX (X = Cu, Fe) although Cu or Fe substitutes for the same atom Ni elements in the NiTi matrix phase. <...> Sui et al. [ 2 ] found that after alloying the Co element, the recovery stress of NiTiNb(Co) was larger than that of NiTiNb SMA, albeit the phase hysteresis was shortened slightly. <...> Saburi et al. [ 3 ] studied the effect of Co, Pd, and Fe elements on the phase transformation ability and found that the starting transformation temperature Ms of the NiTi martensitic phase was increased by Pd alloying but decreased by Co alloying albeit they substituted the same <...>